User equipment may be configured to operate in a connected mode, an idle mode, and, more recently, a power savings mode in accordance with a specification, such as 3GPP 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Architecture enhancements to facilitate communications with packet data networks and applications (Release 11), 3GPP TS 23.682 V11.5.0 (2013-09) (hereinafter TS 23.682). This power savings mode may be particularly useful in certain use cases, such as machine-to-machine devices and the like, although the power savings mode may be used in other use cases as well. The power savings mode (or state) may be adopted by the user equipment, in which case a relatively longer period of inactivity may be implemented at the user equipment. The inactivity may range from one or more minutes to one or more hours (or longer). The user equipment may be configured so that the user equipment is reachable for downlink data only during the time that the user equipment is in a connected mode (for example, a radio resource control (RRC) S1 connected state) and a certain active time period following the connected state, during which the user equipment is reachable for paging. For example, the user equipment may start an active timer after transiting to the idle mode. When the active timer expires, the user equipment changes to another mode, such as a power saving state or other mode. Depending on the user equipment's configuration, the applications of the user equipment may change the user equipment back to a normal, connected operation mode, when an application at the user equipment needs to transfer data.
During the power saving state, the user equipment is not typically reachable by the network. Therefore, the receivers and/or transmitters at the user equipment can be deactivated, so power saving can be achieved. The user equipment may stay in the power saving state for the duration of a timer, such as a periodic tracking area update (P-TAU) timer, such as timer T3412). The periodic tracking area update timer may be negotiated in the network access stratum level during an attach procedure. The periodic tracking area update timer is used by the user equipment to notify the user equipment's availability to the application server in the network. The value of timer T3412 may be sent by the network to the user equipment in an attach accept message, and may be sent in a tracking area update accept message. When the periodic tracking area update timer expires, the user equipment initiates a periodic tracking area procedure and stays in a regular idle/registered state for the period of activity time, which is also negotiated with the network during the attach procedure. When in the regular idle state, the user equipment uses the normal idle mode discontinuous receive (DRX) procedure to save power/battery. This DRX cycle is configurable and can be set to a time up to 2.54 seconds, although other times may be implemented as well.